Wood. 87 (14.) WoQ,d is not used to so great an extent as formerly. Roofs are made of wrought iron; and men-of-war of iron and steel instead of oak: pillars of cast iron : while morticed wheel teeth are almost out of fashion. Brake blocks, too, are made of cast iron, to give a longer time of wear -3 and wooden buffer beams for locomotives are now being discarded. Little then need be said of wood. For pattern-making, as already stated, jbtne, mahogany, cherry, sycamore, lime tree, and walnut are the woods used. JLnglish oak is the best for beams, but American oak is much cheaper, and the latter is used for the framing of railway and traction waggons, and for locomotive buffer beams. Ash is also much employed in waggon work, especially for cart shafts. Mortice teeth are made of beedi or hornbeam* Lignum vitas is of great service for bearings that are , immersed in water as, for example, mth the screw-propeller and some turbines. (See Appendix 77~.,/. 787.) Railway sleepers are rendered very durable by impregnation with creosote or black oil, air being first sucked from the pores of the wood. The creosote is then forced in at great pressure. The following table gives the melting points in degrees Fahrenheit of the principal metals mentioned in this chapter :— Cast Iron __..... 2ioo°P. Wrought Iron____ 3000° * Steel ...____..... 2700° Copper............. Zinc .............. 773°F- Tin................. 44V Gun metal......— 1900". Brass...... 1700° to 1900° v Soft Soldering.—See Appendix K, ^.970, * Castings of * -wrought iron' have been-made^' though the process .is- somewhat intricate, and has not ,been extensively , applied. The method . consists principally in lowering the high melting point of wrought iron by the addition of aluminium. Swedish wrought iron is used,' and from T of its weight of aluminium is mixed with it, "